1 Field of the Invention
The present invention relates to devices intended for supplying electrical power to a light box, that is to say a fluorescent lighting system making it possible for example to illuminate a liquid crystal display screen by transmission.
2. Discussion of the Background
Light boxes are known devices which make it possible to obtain over a relatively extended plane surface, generally of rectangular shape, very uniformly distributed light energy making it possible to examine transparent objects.
A light box can serve for various uses, for example for examining photographic negatives. It is also much used to illuminate, by transparency, the liquid crystal display screens which are increasingly being used instead of cathode-ray tubes.
These liquid crystal screens serve in particular as on-board aircraft displays for providing the pilot with the information he needs, in particular the indications which were previously displayed with the aid of electromechanical indicators.
In a known manner, a light box used to illuminate a liquid crystal display screen in an aircraft consists, for the sake of uniformity of illumination and for luminous intensity, of a battery of fluorescent tubes aligned one against the other and supplied with a large voltage, of the order of 1000 volts, delivered by a high voltage converter.
For simplicity, in the subsequent text the term light box will designate the assembly composed of the battery of fluorescent tubes and of its high voltage supply converter.
To obtain suitably stable lighting, exhibiting no fluctuations irritating the operator, it is necessary to supply the light box with a DC voltage, whose value may lie between 25 and 50 volts but must be suitably stable.
Patent Application GB 2 229 873 discloses a supply circuit for powering a discharge lamp of a car, which operates on the basis of a battery with stable voltage and which comprises protection and warning circuits intended for protecting the assembly, in particular against the consequences of poor operation of the device for igniting the lamp.
It is known that in aeroplanes the DC voltage available for powering the ancillaries is provided by a so-called "28-volt" network. This title is entirely appropriate since, given the various constraints, the actual voltage of this network may vary within considerable limits, lying for example between 12 and 50 volts. It is not therefore possible to power the light box directly from this network.
Furthermore, in order to avoid being confronted with problems of earth loops between primary and secondary of the equipment, there is reason to produce galvanic isolation between the 28-volt network and the light box.
In order simultaneously to obtain appropriate regulation of the supply voltage of the light box and isolation of the latter with respect to the network, it is known practice to use a power supply device comprising a galvanically isolated regulating converter. Such a device is produced for example according to the diagram represented in the appended FIG. 1.
In this figure, a power supply 101 supplies on the one hand a light box 102 and on the other hand an assembly of electronic cards 103 intended for processing the information to be addressed to the LCD display screen (not represented in this figure) which is fixed to the surface of the light box 102. This light box 102 itself comprises in a known manner an assembly of fluorescent tubes 104 powered via a high voltage converter comprising a chopper 105 and a booster transformer 106.
The supply device 101 operates from an input voltage of +28 volts, and which, as has been seen, fluctuates widely. This voltage charges, by way of an isolating diode 108, an assembly of capacitors 109. This diode makes it possible both to protect the equipment against reversals of polarity and to avoid reverse discharging of the capacitors into the supply network during outages of the latter. These capacitors themselves make it possible to guarantee the power supply to the downstream circuits during these possible outages.
The capacitors 109 furthermore make it possible to supply two regulated voltage converters 110 and 111 intended for supplying the light box 102 and the electronic cards 103 respectively. These capacitors are for example of the "flyback" or "forward" known type, whose efficiency is known to lie between 80 and 85%. This efficiency, which might appear to be appropriate in other circumstances, nevertheless exhibits drawbacks with regard to an aircraft where the power available is metered and where the heat originating from the corresponding losses must be dispersed. Furthermore, the converters of this type are relatively expensive and voluminous.
The converter 110 is of the DC/DC regulating and booster type with galvanic isolation. It makes it possible to provide the supply voltage for the light box 102. This supply voltage is for example 50 volts so as to be at the top limit of the variations in the supply voltage originating from the +28-volt DC network.
The converter 111 is of the DC/DC regulating type with galvanic isolation and makes it possible to deliver all the voltages required by the electronic cards 103, for example +5 volts +15 volts and -15 volts.